Juha Javanainen and his coauthors have found a remarkable situation that a two-species mixture of
cold Fermi-Dirac atoms can obtain. As described recently in the Physics Review Focus on-line news
magazine (http://focus.aps.org/v9/st21.html),
ultra-cold atoms placed in a periodic confining
lattice can have partial numbers of atoms at places in the lattice where 'phase kinks' occur.
Studying a two-species FD gas in a 1-D optical lattice coupled to an electromagnetic field with
a phase kink, they show that fractional number of atoms will be located at the nodes in the
potential. The paper describes how experimenters can set up the experiment and detect the
fractional number of atoms at the unique sites in the lattice. The calculation results apply to
2-D and 3-D lattices, as well. Details can be found in the authors' article in Physical Review
Letters
here.